In many applications it may be desirable to amplify an AC signal superimposed to a relatively large DC component. In these cases, an AC amplifier input with this signal through decoupling capacitors is commonly used. In particular, this happens in signal channels for recording data in hard disk drives (HDD).
Two modes of recording data on a HDD are the so-called longitudinal recording and the vertical recording. As far as the AC amplifier is concerned, the main difference between these two techniques is that when using the longitudinal recording technique, a typical spectrum of an AC signal to be amplified, differs relevantly from the spectrum of the corresponding signal when using the vertical recording technique. As may be observed by comparing the two diagrams of FIG. 1, signals used for recording data on a HDD using the vertical recording technique have a non-negligible power content at low frequencies.
Therefore, the pass-band of AC amplifiers optimized for vertical recording may extend to relatively low frequencies, in other words they may have a small low corner frequency (LCF).
Typically, an AC amplifier includes a differential amplifier, as that of FIG. 2. A differential input signal is fed to the inputs INA and INB through respective decoupling input capacitors C. The architecture of this amplifier is very simple, but a sufficiently reduced LCF may be achieved using relatively large decoupling capacitors C.
Unfortunately, in integrated circuits, when the size of the decoupling capacitors C is enlarged, the parasitic capacitances CP1 and CP2 between the plates of the integrated capacitors, and the silicon substrate on which the amplifier is integrated, significantly increase. Thus, also, the input capacitance of the amplifier increases.
In order to keep parasitic capacitances below a maximum acceptable value, these relatively large DC-decoupling capacitors are fabricated with expensive techniques that may require additional masks and fabrication steps.
This drawback may be obviated with the AC amplifier disclosed in the European patent application No. 03425561.2 and depicted in FIGS. 3 and 4. It has been found possible to effectively decouple the DC component of the input signal by employing, for this purpose, decoupling capacitors in a position such that parasitic capacitances associated to the plates of decoupling capacitors do not degrade the input capacitance figure of the stage. The LCF is increased and effects of parasitic capacitances are reduced with the circuit of FIGS. 3 and 4 at the cost of increasing noise and current consumption (for an unchanged overall gain).